This invention relates to a non-cracking polyethersulfone microporous membrane having bulk properties which differ from its surface properties and to a process for preparing the same. The invention relates to the modification of the surface properties of hydrophobic polyethersulfone microporous membranes. In particular, the method includes rendering the hydrophobic surfaces permanently hydrophilic while avoiding causing the membrane to become cracking.
For many porous media applications, a preferred medium must have both certain specific bulk properties and specific surface properties. In many cases, however, a substrate having desirable bulk properties (such as mechanical strength or solvent resistance) has not had appropriate surface properties (such as water wettability, low protein absorbing tendency, thromboresistivity, controlled ion exchange capacity, controlled surface chemical reactivity, and the like). The modification of the surface properties of such substrates has a long history.
One of the oldest methods for modifying surface properties is to coat a porous medium, e.g. a membrane, having desired bulk properties, with an agent having the desired surface properties. This has been done to make an otherwise hydrophobic membrane function as if it is hydrophilic. To deposit a hydrophilic coating, a coating composition including surfactants or other wetting agents is used. This approach to modifying surface properties has generally been found to be undesirable because the resulting coating tends to be temporary and is removed in whole or in part shortly after initial use. Indeed, polymeric membranes treated in this fashion usually can be wetted with water only a single time. In addition, the membranes exhibit a high level of extractables due to removal of the coating. This is unacceptable in many filtration applications, particularly those which entail processing biological fluids which are to be sterilized or subsequently analyzed.
U.S. Pat. No. 4,702,840 discloses a variant of the simple coating composition in that a surface active agent is included in a casting dope which is used to form the basic membrane. This technique usually reduces the rate of extraction of the surfactant, but does not avoid the extraction.
U.S. Pat. No. 4,340,482 discloses a more sophisticated approach in which the surface of a porous membrane formed from hydrophobic fluorine-containing polymers is made hydrophilic by grafting thereto a primary amine, such as glycine. The modified membranes exhibit properties which are undesirable for use with certain materials. For example, the resultant membrane oftentimes has a non-white color and gives off colored extractables during use. Furthermore, the membrane has a tendency to absorb proteins from solution and therefore is unacceptable in applications such as clinical diagnostic assays.
Graft polymerization has been proposed for the modification of the surface characteristics of a polymeric substrate. U.S. Pat. Nos. 3,253,057; 4,151,225; 4,278,777 and 4,311,573 disclose typical examples of such graft polymerizations. However these approaches require high energy ionizing radiation and have not been commercially viable. Moreover, presently available graft polymerization techniques have not succeeded in modifying the entire surface of a porous membrane, i.e. including the portions of the surface located within the pores, while avoiding substantial pore blockage and thereby substantially retaining the porosity of the original membrane.
It would be beneficial to be able to modify the surfaces of porous media by the polymerization of monomers in situ. In this approach the copolymerization of more than one monomer can yield properties not available from commercially available polymers. Also the use of polyfunctional monomers can produce highly insoluble polymers which will be highly insoluble in process fluids.
U.S. Pat. No. 4,618,533 proposes the conversion of a membrane from hydrophobic to hydrophilic by depositing a crosslinked polymer over the surface of a membrane by free radical polymerization of a monomer in a liquid medium. The method requires the use of a free radical initiator in an amount that is at least 1000% more, preferably 5000 to 25,000% more, than the amount of initiator that would be used in a typical free-radical polymerization. The patent asserts that such a high concentration of initiator is required to limit the length of the polymer chains to avoid plugging of the pores of a membrane while uniformly coating the entire exposed pore surface of the substrate polymer. In view of the large amount of initiator required, the resulting membrane must contain a substantial amount of extractable residual initiator. The membrane must require substantial washing before use to reduce this contaminant.
It would be desirable to develop a method for producing a modified membrane surface while avoiding the presence of any free radical initiator.
U.S. Pat. No. 5,468,390 discloses surface modification of a polysulfone membrane by polymerizing a vinyl monomer using ultraviolet light without initiators. The polymerization of a monomer under these conditions is slow because the substrate is opaque to UV light. Moreover, the polymerization can proceed only from the surfaces which actually receive exposure to the UV light. As such, complete polymerization is unlikely and extractables will result.
U.S. Pat. Nos. 4,900,449, 4,964,990, and 5,108,607 describe preparing hydrophilic polyethersulfone membranes by forming a solution of a hydrophobic polymer starting material and adding a high molecular weight (up to 10,000 daltons) polyethylene glycol prior to casting the polymer into a membrane. The high molecular weight polyethylene glycol is responsible for the initial hydrophilicity of the resulting polyethersulfone membrane. However, under process conditions the high molecular weight polyethylene glycol, a known wetting agent, slowly leaches out and contaminates the filtrate.
Gas plasmas are attractive since roll processing equipment is commercially available and penetration of the porous structure by the gas plasma should initiate rapid surface modification throughout the structure. The direct application of a plasma has long been used to modify porous surfaces, usually with the goal of improved wettability. However, the direct treatment of surfaces by plasma is undesirable in the case of membranes where surface ablation and polymer emcrackingment occur. Also, the use of plasma often produces a surface layer which is easily removed by washing. Thus as the surface layer is extracted, the temporarily wettable surface returns to its original, unmodified state.
It has also been reported that plasma has been proven to be an inadequate technique for modifying the inner surface of pores. M. Gato et al, Journal of Membrane Science, 96, (1994) 299, 307, for instance, reports a failure to modify the inner surface of a hollow fiber membrane with plasma because the "plasma could not penetrate into the hollow fiber membrane." Since hollow fiber membranes commonly have lengths in the range of a few inches to several feet, it is possible that the plasmas used were not sufficient to penetrate the full length of the hollow fiber.
A variety of papers disclose the use of a plasma to induce free radical formation in a porous substrate, usually a polypropylene membrane, followed in a separate step by exposure to a monomer to produce a graft polymerization. This two step process has been found to lead to substrates with completely filled void volume which have had some utility as membranes in pervaporation processes. (Yamaguchi, Nakao, Kimura. Macromolecules 1991, 24, 5522-5527.)
In view of the relative ease of performing plasma processes at low costs, it would be desirable to develop a method in which the inner pore surfaces of a membrane are permanently modified using plasma in a single step process without significant loss of void volume.
U.S. Ser. No. 09/246,234, filed on the same day herewith, discloses a method of directly coating the entire, i.e. both internal and external, surface of a porous medium such as a polyethersulfone membrane with a crosslinked polymer by (a) coating the substrate with a solution of one or more polyfunctional polymerizable monomers and (b) exposing the coated porous substrate to a gas plasma which causes polymerization of the monomer in situ over all of the surfaces of the porous structure, the exposure being under conditions which avoid any substantial reduction of the void volume of the porous medium. While the process has been found generally useful with polyethersulfone polymer membranes, when a sufficient amount of polymerized coating is present to produce a permanently wettable product, the resulting membrane has at times exhibited an undesirable behavior. It hasd been found to crack during cutting of the membrane and/or during folding of it to form a pleated cartridge.
Accordingly, it would be highly desirable to provide a composite polyethersulfone membrane having (a) desirable bulk physical strength and chemical resistance, (b) having permanent hydrophilic surface properties over the entire internal surfaces, and also (c) avoiding the cracking problem described above.